Some info on diesel fuel (fungus contamination, shelf life) from BP is entered in the Cruiser's Dictionary WIKI, here Cruisers Dictionary, C-D
scroll down to "Diesel".
When we wound up leaving sv Watermelon in a marina in Australia for more than a year, we figured that the diesel in the tank might have been a bit grotty, and the easiest way to check it out was to pump out from the bottom of the fuel tank a few gallons of diesel into a relatively transparent container. We let it sit, and found that at the bottom of the container was water and gunk. We wound up emptying the tank into containers, siphoning out the stuff that settled, and then filtering the relatively clean diesel back into the tank. Fuel filters are expensive when a cheaper alternative costs little more than time, of which we had plenty.
Shelf life for diesel is only about 12 months, so you probably should put clean diesel into the tank before firing up the engine.
Do you have a diesel mechanic friend? Might be nice for a mechanic to look at the engine.